Enhancement of photocatalytic activity of mesoporous TiO2 by using carbon nanotubes

doi:10.1016/j.apcata.2005.04.057

Applied Catalysis A: General

Volume 289, Issue 2, 10 August 2005, Pages 186-196

https://doi.org/10.1016/j.apcata.2005.04.057 Get rights and content

Abstract

Titanium dioxide/carbon nanotubes (TiO₂/CNTs) composites were prepared with the aid of ultrasonic irradiation. Products of different TiO₂:CNTs molar ratio were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) adsorption analysis, thermogravimetric and differential thermal analysis (TGA–DSC), photoluminescence (PL) and UV–vis spectroscopy measurements (UV–vis). The photocatalytic activity was evaluated by the degradation of acetone and by the detection of the hydroxyl radical (·OH) signals using electron paramagnetic resonance (EPR). It is found that the crystalline TiO₂ is composed of both anatase and brookite phases. The agglomerated morphology and the particle size of TiO₂ in the composites change in the presence of CNTs. The CNTs in the composites are virtually all covered by TiO₂. Other than an increase of the surface area, the addition of CNTs does not affect the mesoporous nature of the TiO₂. Meanwhile, more hydroxyl groups are available on the surface of the composite than in the case of the pure TiO₂. The higher the content of CNTs, there is more effective in the suppression of the recombination of photo-generated e⁻/h⁺ pairs. However, excessive CNTs also shield the TiO₂ from absorbing UV light. The optimal amount of TiO₂ and CNTs is in the range of 1:0.1 and 1:0.2 (feedstock molar ratio). These samples have much more highly photocatalytic activity than P25 and TiO₂/activated carbon (AC) composite. The mechanism for the enhanced photocatalytic activity of TiO₂ by CNTs is proposed.

Introduction

The use of heterogeneous photocatalytic oxidation for water and air purification and remediation is the subject of a wide range of investigations [1]. Among various oxide semiconductor photocatalysts, titania is the most common one because of its strong oxidizing power, absence of toxicity and long-term photostability [2]. Unfortunately, the photocatalytic activity of pure titania is not high enough for industrial purposes [3]. Several methods have been reported to improve the photocatalytic efficiency. These include increasing the surface area of TiO₂, the generation of defect structures to induce space-charge separation, and the modification of TiO₂ with metal(s) or other semiconductor(s) [4], [5], [6]. Another method that might possibly increase the photocatalytic efficiency of TiO₂ is to add a co-sorbent such as silica, alumina, zeolites or clay, but no apparent improvement of photocatalytic efficiency is observed [7], [8], [9]. Therefore, the development of new materials for modifying TiO₂ is urgently needed to increase the photocatalytic activity of TiO₂ for organic pollutant treatment.

Carbon nanotubes (CNTs) attract considerable attention since their discovery [10] due to their special structure, their extraordinary mechanical and unique electronic properties and their potential applications. Their high mechanical strength makes them to be good candidates for advanced composites [11]. They can either be semiconducting, semi-metallic or metallic, depending on the helicity and the diameter of the tube [12]. Such variety opens a promising field in nanoscale electrodevice applications. Their large specific surface area, hollow and layered structures indicate that they can be ideal hydrogen storage materials [13]. Recently, researchers found that CNTs are efficient adsorbents for dioxin, fluoride, lead and cadmium [14], [15], [16], [17]. Thus, CNTs can be used as a promising material in environmental cleaning. CNTs can conduct electrons [18], [19] and have a high adsorption capacity. Anatase TiO₂ is known for its superior photocatalytic ability compared with other photocatalysts. Therefore, the application of CNTs to enhance the photocatalytic activity of TiO₂ is proposed.

Mesoporous TiO₂ is a highly photocatalytically active photocatalyst because it has a high surface-to-volume ratio and offers more active sites for carrying out catalytic reactions [20]. Recently, sonochemical processing has been proven to be a useful technique in the synthesis of mesoporous TiO₂ with high photocatalytic activity [21], [22]. In this study, mesoporous TiO₂ and TiO₂/CNTs composites were prepared by the ultrasound technique. We investigated the effect of CNTs on the microstructure, physicochemical properties, and photocatalytic activity of TiO₂ for the degradation of air pollutant acetone by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) adsorption analysis, thermogravimetric analysis and differential thermal analysis (TGA–DSC), photoluminescence (PL), UV–vis spectroscopy (UV–vis) and electron paramagnetic resonance (EPR). To the best of our knowledge, this is the first report about utilizing TiO₂/CNTs composites in photocatalysis.

Section snippets

Materials

CNTs (multiwall CNTs) were synthesized by the catalytic decomposition of acetylene at 780 °C using Co and Fe catalyst [23], [24]. For the purification, 8 g of raw CNTs were boiled in 400 mL of concentrated nitric acid (65% HNO₃, Merck, Germany) for 40 min to get oxygenated functionalities on the nanotube surface. Then, the CNTs were filtered, washed with 600 mL distilled water for five times to remove acid, and finally dried at 105 °C in an oven (Shel-Lab 1350 FX, Sheldon Manufacturing Inc., USA).

Characterization of the prepared TiO₂ and TiO₂/CNTs composites

To characterize the crystalline structure of the samples, the XRD patterns of TiO₂ and TiO₂/CNTs composites (Fig. 1) are obtained. If one compares these with the pattern of pure CNTs (Supplementary data), one finds that there is no apparent peak in these composites at the positions of 26.0 and 43.4, which are the characteristic peaks for CNTs [27]. Only anatase (JCPDS no. 21-1272) and brookite (JCPDS no. 29-1360) phases attributed to TiO₂ are present. The reason may be that the main peak of

Conclusions

The effect of oxidized CNTs on the crystalline phase and composition, the materials morphology and the physicochemical properties of TiO₂ is investigated using XRD, Raman spectroscopy, BET, SEM, TGA–DSC, XPS, PL, EPR and UV–vis spectroscopy. The results show that the crystalline part of the obtained TiO₂/CNTs composites is composed of anatase and brookite phases, similar to that of the TiO₂. CNTs can affect the agglomerated morphology and the particle size of the TiO₂. Meanwhile, CNTs in the

Acknowledgements

This work was supported by a RGC research grant (No. CUHK4325/03M) of the Hong Kong SAR Government to P.K. Wong and a NSFC research grant (No. 20207002) to Y. Yu. The authors thank Dr. Z. Jia and Mr. X.X. Ding, Institute of Nano Science and Technology, Central China Normal University, for the kind provision of carbon nanotubes and for SEM measurements, respectively.

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Enhancement of photocatalytic activity of mesoporous TiO2 by using carbon nanotubes

Abstract

Introduction

Section snippets

Materials

Characterization of the prepared TiO2 and TiO2/CNTs composites

Conclusions

Acknowledgements

Appl. Catal. B: Environ.

J. Catal.

Chem. Phys. Lett.

Chem. Phys. Lett.

Carbon

Physica B

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Carbon

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Appl. Catal. B: Environ.

Sol. Energy Mater. Sol. Cells

J. Catal.

Chem. Phys. Lett.

J. Solid State Chem.

Appl. Catal. B: Environ.

Appl. Catal. B: Environ.

J. Photochem. Photobiol. A: Chem.

J. Photochem. Photobiol. A: Chem.

Physica B

Appl. Catal. B: Environ.

J. Catal.

Appl. Catal. B: Environ.

Chem. Phys. Lett.

J. Phys. Chem. B

J. Catal.

New J. Chem.

Langmuir

J. Photochem. Photobiol. A: Chem.

Enhancement of photocatalytic activity of mesoporous TiO₂ by using carbon nanotubes

Characterization of the prepared TiO₂ and TiO₂/CNTs composites